Sen. Elizabeth Warren (D-Mass.) endorsed Graham Platner on Thursday in Maine's Democratic primary race for U.S. Senate, backing the challenger over incumbent Gov. Janet Mills ahead of their June showdown.

Platner, described as an oyster farmer and combat veteran, is positioning himself as a populist candidate in the race to determine who will face Sen. Susan Collins (R-Maine) in the November general election. Warren's endorsement makes her the fourth senator to support Platner's candidacy.

The primary contest pits Platner, running as an outsider candidate, against Mills, who currently serves as Maine's Democratic governor. The race is considered significant as it will determine the Democratic nominee to challenge Collins, a longtime Republican incumbent.

Warren cited Platner's military service and background as an oyster farmer in her endorsement, highlighting what she characterized as his populist policy agenda. The endorsement comes as Democratic senators weigh in on competitive primary races across the country ahead of the 2024 election cycle.

The Maine Senate race is expected to be closely watched, given Collins' status as a key swing vote in the closely divided Senate. The Democratic primary winner will face Collins, who has held the seat since 1997, in what could be a competitive general election contest.